1. Field of the Invention
The present invention relates to optical projection systems, and in particular to deep ultra-violet, large-field unit-magnification projection optical systems.
2. Description of the Prior Art
Photolithography is presently employed not only in sub-micron resolution integrated circuit (IC) manufacturing, but also to an increasing degree in advanced wafer-level IC packaging as well as in semiconductor, microelectromechanical systems (MEMS), nanotechnology (i.e., forming nanoscale structures and devices), and other applications.
The present invention, as described in the Detailed Description of the Invention section below, is related to the optical system described in U.S. Pat. No. 4,391,494 (hereinafter, “the '494 patent”) issued on Jul. 5, 1983 to Ronald S. Hershel and assigned to General Signal Corporation, which patent is hereby incorporated by reference. In addition, the present invention as described below is also related to the optical system described in U.S. Pat. No. 5,031,977 (“the '977 patent”), issued on Jul. 16, 1991 to John A. Gibson and assigned to General Signal Corporation, which patent is hereby incorporated by reference.
FIG. 1 is a cross-sectional diagram of an example prior art optical system 8 according to the '494 patent. The optical system described in the '494 patent and illustrated in FIG. 1 is a unit-magnification, catadioptric, achromatic and anastigmatic, optical projection system that uses both reflective and refractive elements in a complementary fashion to achieve large field sizes and high numerical apertures (NAs). The system is basically symmetrical relative to an aperture stop located at the mirror, thus eliminating odd order aberrations such as coma, distortion and lateral color. All of the spherical surfaces are nearly concentric, with the centers of curvature located close to where the focal plane would be located were the system not folded. Thus, the resultant system is essentially independent of the index of refraction of the air in the lens, making pressure compensation unnecessary.
With continuing reference to FIG. 1, optical system 8 includes a concave spherical mirror 10, an aperture stop 11 located at the mirror, and a composite, achromatic plano-convex doublet lens-prism assembly 12. Mirror 10 and assembly 12 are disposed symmetrically about an optical axis 14. Optical system 8 is essentially symmetrical relative to aperture stop 11 so that the system is initially corrected for coma, distortion, and lateral color. All of the spherical surfaces in optical system 8 are nearly concentric.
In optical system 8, doublet-prism assembly 12 includes a meniscus lens 13A, a plano-convex lens 13B and symmetric fold prisms 15A and 15B located on opposite sides of optical axis 14. In conjunction with mirror 10, assembly 12 corrects the remaining optical aberrations, which include axial color, astigmatism, petzval, and spherical aberration. Symmetric fold prisms 15A and 15B are used to attain sufficient working space for movement of a reticle 16 and a wafer 18. The cost of this gain in working space is the reduction of available field size to about 25% to 35% of the total potential field. In the past, this reduction in field size has not been critical since it has been possible to obtain both acceptable field size and the resolution required for the state-of-the-art circuits. However, today this field size reduction is problematic.
FIG. 2 is a cross-sectional diagram of an example prior art optical system 50 according to the '977 patent. System 50 includes a first mirror 52 and a meniscus lens 54 which is desirably of fused silica. System 50 also includes a plano-convex lens 56, desirably of lithium fluoride, and a pair of prisms 60-1, 60-2 made of calcium fluoride. System 50 includes an optical axis 64. Operation of optical system 50 with a source of light exposure (desirably in the ultraviolet range) is analogous to that described in the '494 patent. System 50 has a numerical aperture (NA) of 0.350 and design wavelengths of 249.8 nanometers and 243.8 nanometers. The air-lens and lens-lens transitions in optical system 50 are labeled 1, 2, 3, 4 and 5 across the bottom of FIG. 2.
Unfortunately, for larger NA applications (i.e., NA≧0.435), both the '494 and the '977 systems of a reasonable size cannot achieve high quality imagery over field sizes having a field height larger than 23 mm in the DUV (Deep Ultra-violet) spctrum.